Capacitive sensing is an important technology for modern consumer electronics, appliances, residential control and Internet of things (IoT). Herein, a change in a capacitance value can be used for proximity/gesture detection, material analysis, liquid level sensing, and so on.
Temperature sensing is equally important in modern electronics. Two popular methods of temperature sensing are thermistor and resistance temperature detector (RTD). They measure temperature by correlating a resistance of the sensing element with temperature.
In both capacitorce and resistance based sensing, it needs to convert a change in resistance R value or capacitance C value into a voltage level or time, and then subsequently convert it into a digital value. In the process of converting the change in the resistance R value or the capacitance C value into the digital value, such change is firstly converted into the voltage level via an analog-to-digital converter ADC or converted into the time via a time-to-digital converter TDC, and then subsequently converted into a digital value. In the approach of using time to represent the change in capacitance/resistance value, the time used for RC charging and discharging is a target of interest. This time duration can be measured directly by using a clock signal with known frequency. Or, the capacitance/resistance under investigation can be made as part of a relaxation oscillator (also called stable multivibator) whose oscillation frequency is associated with the resistance value R and the capacitance C value.
In the prior art, the frequency source with a fixed frequency is used to measure the frequency of the relaxation oscillator, so as to measure the change in the resistance R value and the capacitance C value.